This invention relates to catalytic combustion and in particular to a catalyst structure for use in a catalytic combustion process, for example as encountered in gas turbines.
Catalyst bodies for use in such processes may comprise a structure, such as a foam or honeycomb, having through passages supporting, or composed of, a catalyst active for the combustion process. In any combustion process there may be used an assembly of one or more such catalyst bodies. In a catalytic combustion process a combustible mixture of a gaseous fuel and a combustion-supporting gas, e.g. air, at a temperature below that at which autoignition takes place is fed, normally at superatmospheric pressure, typically in the range 2 to 40 bar abs., to the catalyst body assembly wherein combustion takes place giving a hot gas stream. The fuel may be gaseous or liquid at ambient pressure and temperature, but most, if not all, of the fuel should be in the gaseous state at the temperature and pressure at which the combustible mixture is fed to the catalyst body. Examples of suitable fuels include natural gas, propane, naphtha, kerosene, and diesel distillate. At least part of the fuel may be the product of subjecting a hydrocarbon feedstock to catalytic autothermal steam reforming. A process describing the use of catalytic autothermal steam reforming of a hydrocarbon feedstock to produce a gas turbine feedstock is set out in EP 351094.
Touchton et al describe, in the "Journal of Engineering for Power" (Transactions of the ASME) 105 October 1983 pages 797-805, particularly page 799, an assembly of a series of honeycomb catalyst bodies in series wherein the cell density, i.e. number of cells per unit cross section, of the assembly increases in the direction of gas flow therethrough. Thus the honeycomb catalyst bodies of the first two sections of the assembly have 16 and 64 cells per square inch (2.5 and 9.9 cells per cm.sup.2) respectively, while the subsequent sections of the assembly have 100 cells per square inch (15.5 cells per cm.sup.2). This arrangement is though to give more complete catalytic combustion over a range of gas velocities than an arrangement wherein the cell density is the same throughout the length of the assembly. While this arrangement may be satisfactory at relatively low gas velocities, wherein the gas flow through the passages of the catalyst body is laminar, there is some doubt that the use of such a "graded cell" construction is effective at the higher gas velocities encountered in gas turbines wherein the flow through the passages may be turbulent.
Catalytic combustion processes such as those encountered in gas turbine applications are normally operated, at least once the catalyst has "lit-off", at very high gas velocities and this presents problems in maintaining combustion. Typical linear gas velocities through the catalyst body passages during normal operation are in the range 25-150, particularly 50-100, m.s.sup.-1. As the flow rate is increased, the rate at which heat is lost from the catalyst surface to the gas increases. The rate at which fuel is transferred to the catalyst surface also increases as the gas velocity increases. Provided the catalyst is of sufficient activity to burn the fuel, the rate at which heat is released at the catalyst surfaces thus increases as the gas velocity increases. Thus, provided the catalyst is of sufficient activity, the rate of heat release and the rate of heat loss both increase as the gas velocity increases and so the catalyst surface temperature changes little, if at all. As the gas velocity is increased further, eventually a flow rate is reached where the reaction rate cannot be increased and becomes kinetically limited. Further increase in the flow rate increases the heat loss and so the temperature of the catalyst surface falls. This reduces the rate of combustion on the catalyst surface, which results in a further fall in temperature, until a point is reached where combustion can no longer be sustained. The temperature at which combustion can no longer be sustained depends on a variety of factors such as the nature and concentration of the fuel in the combustible mixture, the gas velocity, and the nature and activity of the catalyst. [The switch from mass transfer to kinetic control is not as sharp as might be implied from the above; the net effect is always the sum of the limitations imposed by the mass transfer and the reaction rate].
Available catalysts that are able to tolerate the temperatures normally achieved unfortunately have insufficient activity to enable operation at the gas flow rates normally desired in gas turbine operations; i.e. the desired flow rates are greater than those at which combustion can be sustained. In some cases catalysts that can tolerate the temperatures normally achieved have insufficient activity to enable the catalyst to "light-off", or effect complete conversion, at acceptable preheat temperatures. While there are some catalysts with sufficiently high activity to perform the combustion at lower temperatures, these active catalysts tend to sinter and/or evaporate at the temperatures normally achieved and so the catalyst life is limited.
These problems can be overcome to some extent by increasing the temperature at which the combustible mixture is fed to the combustion apparatus. Thus if the feed temperature is sufficiently high it may be possible to sustain combustion even at high gas flow rates. However it is often not practical to supply the combustible mixture at a high enough temperature. For example it is usually desired to supply the combustible mixture to the combustion apparatus at a temperature in the range 250.degree.-450.degree. C., particularly 300.degree.-400.degree. C. corresponding to the delivery temperature of the compressor producing the pressurised combustible mixture.
It has been proposed in U.S. Pat. No. 4,089,654 to employ an assembly of a series of honeycomb units bearing catalysts of differing activity with the upstream units having a catalyst of greater activity than that of downstream units. In this way the high activity catalyst effects some combustion thus giving a gas stream that is at a sufficiently high temperature that combustion will be sustained when that gas is fed to a subsequent lower activity catalyst that can withstand the normal operating temperature. In that reference, to avoid overheating of the high activity catalyst, provision was made to avoid "line-of-sight" radiation paths from downstream to upstream. For example one unit in the form of a blank disc with a surrounding annulus of the honeycomb catalyst is followed by a second unit of the opposite configuration, viz a central honeycomb catalyst having a surrounding blank annular region.